Main content area

Dehydration Pathways of 1-Propanol on HZSM-5 in the Presence and Absence of Water

Zhi, Yuchun, Shi, Hui, Mu, Linyu, Liu, Yue, Mei, Donghai, Camaioni, Donald M., Lercher, Johannes A.
Journal of the American Chemical Society 2015 v.137 no.50 pp. 15781-15794
1-propanol, adsorption, aluminum, entropy, gases, propylene, silicon, zeolites
The Brønsted acid-catalyzed gas-phase dehydration of 1-propanol (0.075–4 kPa) was studied on zeolite H-MFI (Si/Al = 26, containing minimal amounts of extra framework Al moieties) in the absence and presence of co-fed water (0–2.5 kPa) at 413–443 K. It is shown that propene can be formed from monomeric and dimeric adsorbed 1-propanol. The stronger adsorption of 1-propanol relative to water indicates that the reduced dehydration rates in the presence of water are not a consequence of the competitive adsorption between 1-propanol and water. Instead, the deleterious effect is related to the different extents of stabilization of adsorbed intermediates and the relevant elimination/substitution transition states by water. Water stabilizes the adsorbed 1-propanol monomer significantly more than the elimination transition state, leading to a higher activation barrier and a greater entropy gain for the rate-limiting step, which eventually leads to propene. In a similar manner, an excess of 1-propanol stabilizes the adsorbed state of 1-propanol more than the elimination transition state. In comparison with the monomer-mediated pathway, adsorbed dimer and the relevant transition states for propene and ether formation are similarly, while less effectively, stabilized by intrazeolite water molecules.